Aerosols

Photo by: TMAX

Aerosols are collections of tiny particles of solid and/or liquid
suspended in a gas. The size of particles in an aerosol ranges from about
0.001 to about 100 microns. (A micron is one-millionth of a meter.) The
most familiar form of an aerosol is the pressurized spray can, which can
dispense anything from hair spray to enamel paint to whipping cream.
Aerosols are produced by a number of natural processes and are now
manufactured in large quantities for a variety of commercial uses. They
are also at the root of a number of environmental problems, including air
pollution and destruction of ozone, a natural component of Earth's
atmosphere.

An aerosol spray can.
(Reproduced by permission of

Field Mark Publications

.)

Classification

Aerosols are commonly classified into various subgroups based on the
nature and size of the particles of which they are composed and, to some
extent, the manner in which the aerosol is formed. Although relatively
strict scientific definitions are available for each subgroup, these
distinctions may become blurred in actual practical applications. The most
important of these subgroups are fumes, dusts, mists, and sprays.

Fumes.
Fumes consist of solid particles—ranging in size from 0.001 to 1
micron—suspended in a gas. Probably the most familiar form of a
fume is smoke. Smoke is formed from the incomplete combustion of fuels
such as coal, oil, or natural gas. The particles that make up smoke are
smaller than 10 microns in size.

Dusts.
Dusts also contain solid particles suspended in a gas, usually air, but
the particles are larger in size than those in a fume. They range from
about 1 to about 100 microns in size, although they may be even larger.
Dust is formed by the release of materials such as soil and sand,
fertilizers, coal dust, cement dust, pollen, and fly ash into the
atmosphere. Because of their larger particle size, dusts tend to be more
unstable and settle out more rapidly than do fumes, which do not settle
out at all.

Mists.
Mists are liquid particles—less than about 10 microns in
size—dispersed in a gas. The most common type of mist is that
formed by tiny water droplets suspended in the air, as on a cool summer
morning. If the concentration of liquid particles becomes high enough to
affect visibility, it is then called a fog. A particular form of fog that
has become significant in the last half century is smog. Smog forms when
natural moisture in the air interacts with human-produced components, such
as smoke and other combustion products, to form chemically active
materials.

Sprays.
Sprays form when relatively large (10+ microns) droplets of a liquid are
suspended in a gas. Sprays can be formed naturally, as along an ocean
beach, but are also produced as the result of some human inventions such
as aerosol can dispensers of paints, deodorants, and other household
products.

Sources

About three-quarters of all aerosols found in Earth's atmosphere
come from natural sources. The most important of these natural components
are sea salt, soil and rock debris, products of volcanic emissions, smoke
from forest fires, and solid and liquid particles formed by chemical
reactions in the atmosphere.

Volcanic eruptions are major, if highly irregular, sources of atmospheric
aerosols. The eruptions of Mount Hudson in Chile in August 1991 and Mount
Pinatubo in the Philippines in June 1991 produced huge volumes of aerosols
that had measurable effects on Earth's atmosphere.

Words to Know

Acid rain:
A form of precipitation that is significantly more acidic than neutral
water, often produced as the result of industrial processes.

Chlorofluorocarbons (CFCs):
A group of organic compounds once used widely as propellants in
commercial sprays but regulated in the United States since 1987 because
of their harmful environmental effects.

Dust:
An aerosol consisting of solid particles in the range of 1 to 100
microns suspended in a gas.

Electrostatic precipitator:
A device for removing pollutants from a smokestack.

Fume:
A type of aerosol consisting of solid particles in the range 0.001 to 1
micron suspended in a gas.

Micron:
One-millionth of a meter.

Mist:
A type of aerosol consisting of droplets of liquid less than 10 microns
in size suspended in a gas.

Ozone layer:
A region of the upper atmosphere in which the concentration of ozone is
significantly higher than in other parts of the atmosphere.

Smog:
An aerosol form of air pollution produced when moisture in the air
combines and reacts with the products of fossil fuel combustion.

Smoke:
A fume formed by the incomplete combustion of fossil fuels such as
coal, oil, and natural gas.

Spray:
A type of aerosol consisting of droplets of liquid greater than 10
microns in size suspended in a gas.

Stack gases:
Gases released through a smokestack as the result of some
power-generating or manufacturing process.

The remaining atmospheric aerosols result from human actions. Some, such
as the aerosols released from spray-can products, go directly to form
aerosols in the atmosphere. Others undergo chemical changes; for
example, oxides of nitrogen and sulfur are produced during the combustion
of fossil fuels such as coal and oil. These oxides may be converted to
liquid or solid nitrates and sulfates, which are then incorporated into
atmospheric aerosols.

Physical properties

The physical and chemical properties of an aerosol depend to a large
extent on the size of the particles that make it up. When those particles
are very large, they tend to have the same properties as a macroscopic
(large size) sample of the same material. The smaller the particles are,
however, the more likely they are to take on new characteristics different
from those of the same material in bulk.

Aerosols tend to coagulate, or to collide and combine with each other to
form larger bodies. A cloud, for example, consists of tiny droplets of
water and tiny ice crystals. These particles move about randomly within
the cloud, colliding with each other from time to time. As a result of a
collision, two water particles may adhere (stick) to each other and form a
larger, heavier particle. This process results in the formation of
droplets of water or crystals of ice heavy enough to fall to Earth as
rain, snow, or some other form of precipitation.

Synthetic production

The synthetic production of aerosols for various commercial purposes has
become such a large industry that the term aerosol has taken on a new
meaning. Average citizens who know little or nothing about the scientific
aspects of aerosols recognize the term as referring to devices for
dispensing a wide variety of products, including hair spray, furniture
polish, and spray paint.

The concept of aerosol technology is relatively simple. A spray can is
filled with a product to be delivered (such as paint), a propellant, and
sometimes a carrier to help disperse the product. Pressing a button on the
can releases a mixture of these components in the form of an aerosol.

Most aerosols, however, are actually more complex than this simple
description. An aerosol pesticide, for example, must be formulated in such
a way that a precise amount of poison is released, enough to kill pests,
but not so much as to produce an environmental hazard. Similarly, an
aerosol throat spray must deliver a carefully measured quantity of
medication.

The production of commercial aerosols fell slightly in the late 1980s
because of concerns about the ozone (see Environmental factors below)
and other environmental effects. By 1992, however, their manufacture had
rebounded. In that year 990 million container units (bottles and cans) of
personal aerosol products and 695 million container units of household
products were manufactured.

Combustion aerosols

Aerosol technology has made possible vastly improved combustion systems,
such as those used in fossil-fueled power generator plants and in rocket
engines. The fundamental principle involved is that any solid or liquid
fuel burns only at its surface. The combustion of a lump of coal proceeds
relatively slowly because inner parts of the coal cannot begin to burn
until the outer layers are burned off first.

The rate of combustion can be increased by dividing a lump of coal or a
barrel of fuel oil into very small particles, the smaller the better.
Power-generating plants today often run on coal that has been pulverized
to a dust or on oil that has been converted to a mist. The dust or mist is
then thoroughly mixed with an oxidizing agent, such as air or pure oxygen,
and fed into the combustion chamber. The rate of combustion of such
aerosols is many times greater than would be the case for coal or oil in
bulk.

Environmental factors

A number of environmental problems have been connected to aerosols, the
vast majority of them associated with aerosols produced by human
activities. For example, smoke released during the incomplete combustion
of fossil fuels results in the formation of at least two major types of
aerosols that may be harmful to plant and animal life. One type consists
of finely divided carbon released from unburned fuel. This soot can damage
plants by coating their leaves and reducing their ability to carry out
photosynthesis (using light to break down chemical compounds). It can also
clog the alveoli—air sacs in human lungs—and interfere with
a person's respiration.

A second type of harmful aerosol is formed when smokestack gases, such as
sulfur dioxide and nitrogen oxides, react with oxygen and water vapor in
the air to form sulfuric and nitric acids, respectively. Mists containing
these acids may be carried hundreds of miles from their original source
before conglomeration occurs and the acids fall to Earth as acid rain.
Considerable disagreement exists about the precise nature and extent of
the damage caused by acid rain. But there seems to be little doubt that in
some locations it has caused severe harm to plant and aquatic life.

Ozone depletion.
A particularly serious environmental effect of aerosol technology has
been damage to Earth's ozone layer. This damage appears to be
caused by a group of compounds known as chlorofluorocarbons (CFCs) which,
for more than a half century, were by far the most popular of all
propellants used in aerosol cans.

Scientists originally felt little concern about the use of CFCs in aerosol
products because they are highly stable compounds at conditions
encountered on Earth's surface. They have since learned, however,
that CFCs behave very differently when they diffuse into the upper
atmosphere and are exposed to the intense solar radiation present there.

Under those circumstances, CFCs decompose and release chlorine atoms that,
in turn, react with ozone in the stratosphere (the atmospheric region
approximately 7 to 31 miles above Earth's surface). As a result,
the concentration of ozone in portions of the atmosphere has been steadily
decreasing. This change could prove to be very dangerous, since
Earth's ozone layer absorbs ultraviolet radiation from the Sun and
protects living things on our planet from the harmful effects of that
radiation.

Technological solutions.
Methods for reducing the harmful environmental effects of aerosols such
as those described above have received the serious attention of scientists
for many years. Consequently, a number of techniques have been invented
for reducing the aerosol components of things like stack gases. One
device, the electrostatic precipitator, is based on the principle that the
particles of which an aerosol consists (such as unburned carbon in stack
gases) carry small electrical charges. By lining a smokestack with charged
metal grids, the charged aerosol particles can be attracted to the grids
and pulled out of the emitted smoke.

Efforts aimed at solving the CFC/ozone problem have not yet been as
successful as those used to combat other forms of air pollution. Chemists
have developed and tested a number of substitutes for CFCs as aerosol
propellants. One group of special interest has been the
hydrochlorofluorocarbons (HCFCs), CFC-like compounds that also contain
hydrogen atoms. Most CFC-substitutes tried so far, however, are very
expensive, not efficient enough as a propellant, or equally harmful to the
environment.

The dangers of aerosol sniffing

Another risk associated with commercial aerosols is their use as
recreational drugs. Inhalation of some consumer aerosol preparations may
produce a wide variety of effects, including euphoria, excitement,
delusions,
and hallucinations. Repeated sniffing of aerosols can result in addiction
that can cause intoxication, damaged vision, slurred speech, and
diminished mental capacity.

Thia is such a faciniating website. It has helped me alot on how aerosol cna be very harmful on plants and animals. It has also helped me on information for a paper I am writing about the harmful affects of aerosol. I would truly recommend this website who is truly intrested in the things that harm the Earth. :)

Thank u so much. This info was very helpful in my research on plant nutrient absorption. However, I didn't quite get your view about how aerosols can be artifically formed; u mean its just by storing/containing a content in a pressurized can? That's all?